Dual frequency band,polarization diverse tracking feed system for a horn antenna

ABSTRACT

THE APPARATUS OF THE PRESENT INVENTION CONSTITUTES A FEED TO A SINGLE HORN-TYPE ANTENNA PROVIDING ONE FREQUENCY AND FOR COMMUNICATIONS AND MONOPULSE TRACKING AND A HIGHER FREQUENCY BAND FOR HIGH POWER TRANSMITTING MORE PARTICULARLY, THE HIGHER FREQUENCY BAND IS OF THE ORDER OF 50-100 PERCENT HIGHER THAN THE FREQUENCY BAND FOR THE RECEIVE AND TRACKING FREQUENCIES. IN THIS LATTER FREQUENCY BAND, THE ANTENNA IS CAPABLE OF RECEIVING COMMUNICATIONS AND HAVING MONOPULSE TRACKING CAPABILITY WITH POLARIZATION DIVERSITY, I.E., OTHOGONAL LINEAR, ORTHOGONAL CIRCULAR, ROTATABLE LINEAR OR ARBITRARY ELLIPTICAL POLARIZATIONS. CROSS-POLARIZATION OF ALL PATTERNS IS VERY LOW AND CROSS-TALK BETWEEN AZIMUTH AND ELEVATION TRACKING CHANNELS IS SUBSTANTIALLY ZERO. ALSO, HIGH EFFICIENCY   TRACKING CAN BE ACHIEVED OVER A BROAD FREQUENCY RANGE WITHOUT RETUNING OF COMPONENTS SUCH AS COUPLERS OR FILTERS.

J. s. AJIOKA 3,566,309

, POLARIZATION DIVERSE TRACKING 'Feb.23, 1971 DUAL FREQUENCY BAND FEEDSYSTEM FOR A HORN ANTENNA 5 Sheets-Sheet 1 Filed Feb.- 24, 1969 max/Wag.4724455 IAf/oA A AV BM 24, MM

J. S. AJIOKA 3,566,309 SETRACKI D, POLARIZATION DIVER Feb. 23, 1971 DUALFREQUENCY BAN FEED SYSTEM .FOR A HORN ANTENN 5 Sheets-Sheet 2 Filed Feb;24, 19 9 AQN 3 Feb. 23,1971 J s. AJIOKA 3,566,309

File d Feb; 24, 1969 DUAL FREQUENCY BAND; POLARIZATION DIVERSE TRACKINGFEED SYSTEM FOR A HORN ANTENNA 5 Sheets-Sheet 5 f V Male/a Man 4a xv!WWI/2m due/7477a Feb. 23,1971 J. 5. AJIOKA 1 3,566,309

I v DUAL FREQUENCY BAND, POLARIZATION DIVERSE TRACKING FEED SYSTEM FOR AHORN ANTENNA Filed Feb. 24, 1969 I r a Sheets-Sheet 4 741 fa) rza/ 0/459 7410/ I Y rf" v i *4! 2,71] "2; 72- @s) Feb. 23, 1971 N J. 5. AJIOKAI 3,566,309

DUAL FREQUENCY BAND, POLARIZATION DIVERSE TRACKING FEED SYSTEM FOR AHORN ANTENNA I Filed Feb. 24,1969 '5 Sheets-Sheet 5 A4005 calms/1V4 7/0:Ia: 42/4407 54:04 .Eia 6. 44/0 45144 7/0 iezae P477585 :ae #ae/z- 71 as)A United States Patent Oifice 3,566,309 Patented Feb. 23, 1971 U.S. Cl.333-6 12 Claims ABSTRACT OF THE' DISCLOSURE The, apparatus of thepresent invention constitutes a feed to a single horn-type antennaproviding one frequency band for communications and monopulse trackingand a higher frequency band for high power transmitting. Moreparticularly, the higher frequency band is of the order of 50-100percent higher than the frequency band for the receive and trackingfrequencies. In this latter frequency band, the antenna is capable ofreceiving communications and having monopulse tracking capability withpolarization diversity, i.e., orthogonal linear, orthogonal circular,rotatable linear or arbitrary elliptical polarizations.Cross-polarization of all patterns is very low and cross-talk betweenazimuth and elevation tracking channels is substantially zero. Also,high efliciency tracking can be achieved over a broad frequency rangewithout retuning of components such as couplers or filters.

BACKGROUND OF THE INVENTION A contemporary dual frequency feed systemwith monopulse capability for use as a feed to a horn-type antenna isdescribed in the NASA document, T ELSTAR I NASA SP32, vol. 2, June 1963,pp. 1283-1307. In this method, the tracking signal in the receive band(lower frequency band) uses the TM mode and the TE modes which arelightly coupled to the main circular waveguide that feeds the horn (seepage 1303 of the above document). The disadvantages of this scheme are:

(a) Narrow bandwidth capability There is loss in communication signaland addition of noise proportional to the amount of the fundamental TEmode coupled out for tracking if tracking is done on the communicationsignal. This is because the tracking signal (TE mode from the coupler)is abstracted before the signal is preamplified. If the tracking is doneon a beacon signal in the same general band as communications, narrowband-pass filters that pass only the beacon signal to the coupler andreject the communication frequencies are required. This contributes tovery narrow band tracking capability. In addition, two sets of couplers(orthogonal and laterally displaced couplers, shown on page 1303 of theabove document) are required to get all the tracking information. Thephasing between these orthogonal and laterally displaced couplers,together with their phases relative to the TE mode is very critical.This method of extracting tracking information is inherently very narrowband; i.e., it is not a simple adjustment to change to a difierentbeacon or tracking frequency. Even for single frequency operation, manyphase trimmers and attenuators are required.

(b) Restricted to circular polarization This method is generally onlyapplicable to circularly polarized signals. For linear polarization,there is no tracking information in the plane normal to the axis ofpolarization resulting in target loss (see pp. 12941295 of abovedocument). Even in the case of circular polarization, a 3 db loss due tocross polarization is incurred, since the TM, is radially polarized.

(c) Non-separable azimuth and elevation tracking The azimuth andelevation tracking signals are not independent. That is, the trackingerror signal in azimuth depends on the elevation angle. To circumventthis problem, an additional coordinate converter is required (see pp.1297-1298 of above document).

SUMMARY OF THE INVENTION The present invention eliminates all of thedisadvantages of the contemporary method cited. For purposes ofexplanation only, a transmit frequency in the 6 gHz. band and a receiveand track frequency in the 4 gHz band is used. In accordance with thepresent invention, a feed system is provided which includes a modelauncher that comprises a square array of four 4 gHz orthogonalpolarization mode transducers With a 6 gHz launcher at the center of thearray. Each quadrant of the quadarray has a horizontal polarization portand a vertical polarization port constituting a total of eight 4 gHz.output ports/These eight ports are excited by a hybrid arihmetic networkin the proper amplitudes and phases to achieve the desired independentmodes. The square quadarray transitions to a common circular waveguidewhich constitutes the interface between the feed system and. the antennasystem. This common circular waveguide is of a diameter that is largeenough to support all the desired modes but is below cutoff forundesired higher order modes. A 6 gHz. transmit port comes in with aridge-loaded circular wave guide at the center of the quadarray. Thewaveguide ridges extend and become common to the common walls of thequadarray to form a tapered-ridge transition to the common multi-modecircular waveguide. This taperedridge transition launches the 6 gHz.transmit wave into the common large circular waveguide with littlecoupling back into the 4 gHz. components. Additional decoupling isafforded by 6 gHz. band-reject filters in each of the 4 gHz. quadrants.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 illustrates a partiallycut-away perspective view of the quadarray mode launcher of the dualfrequency band feed system of the present invention;

FIG. 2 shows a longitudinal section of the quadarray mode launcher ofFIG. 1;

FIG. 3 shows a front view of the quadarray mode launcher of FIG. 1,together with the placement of transmit frequency reject filters in thereceive waveguides;

FIG. 4 is a schematic representation of a hybrid network to interfacewith the eight 4 gHz. waveguide arms of the quadarray mode launcher ofFIGS. 1 and 3;

FIG. 5 illustrates 4 gHz. mode excitation diagrams in the quadarray modelauncher of FIGS. 13;

FIG. 6 illustrates mode combinations for azimuth error and elevationerror patterns for horizontal polarization;

FIG. 7 illustrates mode combinations for azimuth error and elevationerror patterns for vertical polarization; and

FIG. 8 illustrates schematic diagrams of hybrid networks for generatingazimuth and elevation error signals for horizontal and verticalpolarization, together with attenuators and phase trimmers to properlybalance the modes in amplitude and phase for boresighting the antennasystem.

Referring now to FIGS. 1, 2 and 3 of the drawings, there are showncut-away, perspective and sectional views of the quadarray mode launcherassembly of the feed system of the present invention. The disclosed feedsystem, by Way of example, is adapted to transmit horizontally andvertically polarized 6 gHz. signals, to receive horizontally andvertically polarized 4 gHz. signals and to extract tracking informationfrom either or both of these latter signals. In particular, thequadarray mode launcher includes a square array of four 4 gHz.orthogonal polarization mode transducers 10, 11, 12, 13 with a circularwaveguide 15 extending through the center thereof having ridges 16, 17,18, 19 spaced in quadrature therethrough and coinciding with the commonwalls of the mode transducers 10, 11, 12, 13. A 6 gHz. orthogonalpolarization transducer 20 feeds the ridge-loaded circular waveguide 15from the left extremity, as viewed in the drawings, and includes awaveguide arm 21 which proyides a transmit input for horizontallypolarized 6 gHz. signals and a waveguide arm 22 which provides atransmit input for vertically polarized 6 gHz. signals.

Each 4 gHz. orthogonal polarization mode transducers 10, 11, 12, 13 ofthe quadarray has a horizontal polarization port and a verticalpolarization port constituting a total of eight 4 gHz. output ports.Waveguide arms 24, 25, 26, 27 connect the horizontal polarization portsof mode transducers 10, 11, 12, 13, respectively with hte correspondingsignals designated as A B C and D Similarly, waveguide arms 28, 29, 30,31 connect to the vertical polarization ports of mode transducers 10,11, 12, 13 respectively with the corresponding signals designated as A BC and D The waveguide arms 24-31 include 6 gHz. reject filters 32-39,respectively, FIG. 3, and connect to input terminals -47, respectively,of the hybrid network for 4 gHz. mode excitation, FIG. 4.

The quadarray of mode transducers 10, 11, 12, 13 transitions to a commoncircular waveguide 48 of a diameter large enough to support all thedesired modes but below cutoff for undesired higher order modes. Theridges 16, 17, 18, 19 of the ridge-loaded circular waveguide 15 extendinto tapered ridges 50, 51, 52, 53, respectively, which extend from thecommon walls of the quadarray of mode transducers 10, 11, 12, 13 to theopposite extremity of common circular waveguide 48 thereby to provide atapered-ridge transition from the mode transducers 10-13 to the commoncircular waveguide 48. This tapered ridge transition launches the 6 gHz.transmit wave into the common large circular waveguide 48 with littlecoupling back into the 4 gHz. orthogonal polarization mode transducers10-13. Additional decoupling is afforded by the 6 gHz. band-rejectfilters 32-39 in the waveguide arms 24-31. For the present case, thecommon circular multimode waveguide 48 transitions to a circularcrosssection of 5.85 and may include an output flange to mate with theinput of a horn-reflector antenna, not shown. Conductive sheet isdisposed normal to the axis of common circular waveguide 48 between theouter wall of the quadarray of mode transducers 10, 11, 12, 13 and thecircular waveguide 48 at the junction thereof to prevent leakage ofmicrowave energy therefrom.

The desired modes are launched into the common circular waveguide 48 byproper excitation of the eight 4 gHz. waveguide arms 24-31 of thequadarray of orthogonal polarization transducers 10-13, as shown in FIG.5. Each quadrant of the orthogonal polarization transducers 10-13 isdesignated by A, B, C, D, respectively, and the horizontal and verticalpolarization ports are designated by the subscripts H and V,repsectively. Using this notation, following are the algebraic notationsfor the TE 21 01 oi, nv and llH I110de$ in the common circular waveguide48:

In the above relations 1-6 a convention is used wherein an electricfield vector pointing to the right, as viewed in the drawing, isconsidered positive for the horizontally polarized field components andan electric field vector 4 pointing up, as viewed in the drawing isconsidered positive for the vertically polarized components. In theevent that coaxial arms are employed in lieu of the waveguide arms24-31, the polarization would be the same as the orientation of thecoaxial arm instead of being orthogonal to it as in the case ofrectangular waveguide.

Referring to FIG. 4, there is shown a schematic diagram of a hybridnetwork for 4 gHz. mode excitation of the quadarray mode launcher ofFIGS. l-3 in accordance with the relations '1-6, FIG. 5. In thisschematic, a magic tee symbol 50 is employed wherein x, y signals areapplied to input ports 51, 52 respectively, shown horizontally,whereupon the difference signal A(xy) appears at the difference outputport 53, shown vertically, and the summation signal 2(x+y) appears atthe sum output port 54, shown pointing forward, in perspective. Aspreviously specified, the inputs 40-47 of the hybrid network areconnected to the waveguide arms 24-31, respectively, whereby the signalsA B C D A B C and D are applied thereto. The input terminals 40, 41 areconnected to the input ports of a magic tee 55, the input terminals 42,43 are connected to the input ports of a magic tee 56, the inputterminals 44, 45 are connected to the input ports of a magic tee 57 andthe input terminals 46, 47 are connected to the input ports of a magictee 58. The summation output ports of magic tees 55, 56 and 57, 58 areconnected, respectively, to the input ports of magic tees 59, 60 wherebythe summation output ports 61, 62 thereof constitute receive outputports for horizontally and vertically polarized signals, respectively.That is, the horizontally polarized signal H+ H+ H+ H) is available atthe receive output port 61 if a signal having horizontally polarizedcomponents is received and the vertically polarized signal E(A +B +C +Dis available at the receive output port 62 if a signal having verticallypolarized components is received.

The difierence output ports of magic tees 55, 56 and 57, 58 areconnected, respectively, to the input ports of magic tees 64, 65, therespective difference output ports of which are terminated by impedances66, 67. The difference output port of magic tee 59, together with thesummation output port of magic tee 65, are connected to input ports of amagic tee 68 whereby the difference output port thereof provides adominant rectangular waveguide mode signal representative ofcorresponding to TE (0) in common circular wave guide 48 (Relation 2)and the summation output port thereof provides a TE signalrepresentative of corresponding to TE in common circular waveguide 48(Relation 3). In addition, the summation output port of magic tee 64,together with the difference output port of magic tee 60 are connectedto the input ports of a magic tee 70 whereby a dominant rectangularwaveguide mode signal representative of corresponding to TM in thecommon circular waveguide 48 (Relation 4) is available at the differenceoutput port thereof and a dominant rectangular waveguide mode signalrepresentative of corresponding to TE (45) in the common circularwaveguide 48 (Relation 1) is available at the summation output portthereof.

Referring to FIG. 6 there is shown the mode combinations for azimutherror and elevation error field distributions 72, 74, respectively, whena horizontally polarized signal is received. -In particular, the TE (0)field less the TE field provides the field pattern 72 which will have ahorizontal null plane represented by dashed line 73 through the centerthereof when the common circular waveguide 48 and associated horn orreflector (not shown) is on target.) The electric field vectors onopposite sides of the null plane 73 are generally horizontal and ofopposite directions, whereby there is no net signal in a balancedsituation. Since the electric field vectors of pattern 72 can bebalanced by moving the common circular waveguide 48 and the associatedhorn or reflector antenna (not shown) vertically, the difference fieldTE (0)TE constitutes an elevation error signal for a horizontallypolarized receive signal. Similarly, the difference field TM TE (45)constitutes a horizontally polarized field pattern 74 having a verticalnull plane 75 through the center thereof, whereby the patterns 72, 74provide elevation and azimuth error indications, respectively, for ahorizontally polarized receive signal.

Referring to FIG. 7 there is shown the mode combinations for azimutherror and elevation error patterns 76, 78, respectively, when avertically polarized signal is received. In particular, the TM fieldplus the TE (45) field provides the pattern 76 which will have ahorizontal null plane represented by dashed line 77 through the centerthereof when the common circular waveguide 48 and associated horn orreflector system (not shown) is on target. That is, the electric fieldvectors of pattern 76 are vertically polarized and of opposite directionon opposite sides of the null plane 77 whereby there is no net signal ina balanced situation. Since the electric field vectors of pattern 76 canbe balanced; i.e., made equal and opposite, by moving the commoncircular waveguide 48 and associated horn or reflector system (notshown) vertically, the summation field TM +TE (45 constitutes anelevation error signal for a vertically polarized receive signal.Similarly, the summation field TE (0)+TE constitutes a verticallypolarized pattern 78 having a vertical null plane 79 through the centerthereof whereby the patterns 76, 78 provide elevation and azimuth errorindications, respectively, for a vertically polarized receive signal.

Referring to FIG. 8 there is shown a schematic of a hybrid network forcombining modes in a manner to achieve azimuth and elevation trackingsignals for horizontal and vertical polarized receive signals by sensingthe electric field patterns 72, 74, 76, 78 of FIGS. 6 and 7. In thisschematic the same notation for a magic tee is employed as in the caseof FIG. 4. In particular, the difference output port of magic tee 68,FIG. 4, is connected through a phase shifter 80 and an attenuator 81 toan input port of a magic tee 82, the remaining input port of which isconnected to the summation output port of magic tee 68, FIG. 4. Thedifference and summation output ports of magic tee 82 then sense thepresence of patterns 72 and 78, thereby providing an elevation errorsignal at a port 83 for a horizontally polarized receive signal and anazimuth error signal at a port 84 for vertically polarized receivesignal, respectively. In addition, the difference output port of magictee 70, FIG. 4, is connected through a phase shifter 85 and attenuator86 to an input port of a magic tee '87, the remaining input port ofwhich is connected to the summation output port of magic tee 70, FIG. 4.The ditference and summation output ports of magic tee 87 then sense thepresence of patterns 74 and 76, thereby providing an azimuth errorsignal at a port 88 for a horizontally polarizel receive signal and anelevation error signal at a port 89 for a vertically polarized receivesignal. The purpose of the attenuators 81, 86 is to equalize the signalsapplied to the respective inputs of the magic tees 82, 87 andaccordingly is to be placed ahead of the input port receiving thestrongest signal. The phase shifter 80, 85, on the other hand, may beinserted ahead of either input port of the magic tees 82, 87.

In the above description, frequency ranges and dimen sions have beengiven by way of example only.

What is claimed is:

1. A feed system for receiving and tracking over a first frequency bandand for transmitting over a second frequency band that is substantiallyhigher than said first frequency band, said feed system comprising afirst waveguide with quadrangular symmetry about the longitudinal axisthereof, having a cutoff intermediate said first and second frequencybands, said first waveguide having longitudinal ridges disposed inquadrature along the inner surface thereof; means coupled to oneextremity of said first Waveguide for transmitting electromagneticenergy within said second frequency band therethrough; first, second,third and fourth orthogonal polarization transducers for operation oversaid frequency band, each of said first, second, third and fourthorthogonal polarization transducers having a horizontal and a verticalpolarization output and being disposed about said first waveguide withcommon walls in alignment with said ridges thereof with input ports in acommon plane on the side thereof opposite from said one extremity ofsaid first Waveguides; a common circular waveguide capable ofpropagating electromagnetic energy Within both said first and secondfrequency bands disposed symmetrically about said longitudinal axis ofsaid first Waveguide at the extremity thereof opposite from said oneextremity adjacent to said first, second, third .and fourth orthogonalpolarization transducers; a tapered-ridge transition extending from eachof said common walls of said first, second, third and fourth orthogonalpolarization transducers and said longitudinal ridges in said firstwaveguide to the circular cross section of said common circularwaveguide; and means coupled to said horizontal and verticalpolarization outputs from said first, second, third and fourthorthogonal polarization transducers for receiving polarization diversetracking and communication signals within said first frequency band.

2. The feed system for receiving and tracking over a first frequencyband and for transmitting over a second frequency band that issubstantially higher than said first frequency band as defined in claim1, wherein means for rejecting electromagnetic energy within said secondfrequency band is interposed in series with each horizontal and verticalpolarization output of said first, second, third and fourth orthogonalpolarization transducers.

3. The feed system for receiving and tracking over a first frequencyband and for transmitting over a second frequency band that issubstantially higher than said first frequency band as defined in claim1, wherein said means coupled to one extremity of said first waveguidefor transmitting electromagnetic energy Within said second frequencyband therethrough additionally includes means for polarizing saidtransmitted electromagnetic energy in a selected one of a plurality ofpolarization states.

4. A feed system for receiving and tracking over a first frequency bandand for transmitting over a second frequency band that is substantiallyhigher than said first frequency band, said feed system comprising afirst circular Waveguide of predetermined diameter having a cutoffintermediate said first and second frequency bands, said first circularwaveguide having longitudinal ridges disposed in quadrature along theinner surface thereof; means coupled to one extremity of said firstcircular Waveguide for transmitting electromagnetic energy within saidsecond frequency band therethrough; first, second, third and fourthorthogonal polarization transducers, each with a horizontal and avertical polarization output, for operation over said first frequencyband and said first, second, third and fourth orthogonal polarizationtransducers being disposed about said first circular Waveguide withcommon walls in alignment with said ridges thereof and input ports in acommon plane on the side thereof opposite from said one extremity ofsaid first circular waveguide; 21 common circular waveguide of adiameter substantially larger than said predetermined diameter disposedsymmetrically about the longitudinal axis of said first circularwaveguide at the extremity thereof opposite from said one extremity andadjacent to said first, second, third and fourth orthogonal polarizationtransducers; a tapered-ridge transition extending from each of saidcommon walls of said first, second, third and fourth orthogonalpolarization transducers and said longitudinal ridges in said firstcircular waveguide to the inner periphery of said common circularwaveguide, and means coupled to said horizontal and verticalpolarization outputs from said first, second, third and fourthorthogonal polarization transducers for receiving polarization diversetracking and communication signals within said first frequency band.

5. The feed system for receiving and tracking over a first frequencyband and for transmitting over a second frequency band that issubstantially higher than said first frequency band as defined in claim4 wherein said polarization diverse tracking and communication signalswithin said first frequency band include the TE (45), TE21(0O), TEOI,TMol, TEu (vertical) and TEn (horizontal) modes.

6. A feed system for receiving and tracking over a first frequency bandand for transmitting over a second frequency band, said second frequencyband being substantially higher than said first frequency band, saidfeed system comprising a first waveguide having quadrangular symmetryabout a longitudinal axis thereof and a cut-off intermediate said firstand second frequency bands, said first waveguide having longitudinalridges disposed in quadrature along the inner surface thereof; anorthogonal polarization transducer for operation over said secondfrequency band coupled to one extremity of said first waveguide; first,second, third and fourth conductive cubicles disposed about said firstwaveguide with common walls in alignment with said ridges thereof, saidcubicles being open on the side thereof opposite from said one extremityof said first waveguide; a common circular waveguide of a diametercapable of supporting basic modes of said first and second frequencybands disposed symmetrically about said longitudinal axis of said firstwaveguide adjacent to said open side of said first, second, third andfourth cubicles; means coupled to each of said first, second, third andfourth cubicles for separately coupling to horizontal and verticalpolarized electromagnetic energy therein, and a tapered-ridge transitionextending from each of said common walls of said first, second, thirdand fourth cubicles and said longitudinal ridges in said first waveguideto the inner periphery of said common circular waveguide.

7. The feed system for receiving and tracking over a first frequencyband and for transmitting over a second frequency band, as defined inclaim 6 wherein said tapered-ridge transition occurs over a distancegreater than the diameter of said common circular waveguide.

8. The feed system for receiving and tracking over a first frequencyband and for transmitting over a second frequency band as defined inclaim 6, additionally including means coupled to said means coupled toeach of said first, second, third and fourth cubicles for separatelycoupling to horizontally and vertically polarized electromagnetic energytherein for receiving TE (45), TE (0), TE TM TE (horizontal) and TE(vertical) modes within said common circular waveguide as dominant modesin respective separate waveguides.

9. The feed system for receiving and tracking over a first frequencyband and for transmitting over a second frequency band as defined inclaim 8, additionally including means coupled to said separatewaveguides corresponding to said TE (0) and TE modes in said commoncircular waveguide for generating "an elevation error signal for ahorizontally polarized signal in said common circular waveguide.

10. The feed system for receiving and tracking over a first frequencyband and for transmitting over a second frequency band as defined inclaim 8, additionally including means coupled to said separatewaveguides corresponding to said TE and TE modes in said common circularwaveguide for generating an azimuth error signal for a verticallypolarized signal in said common circular waveguide.

11. The feed system for receiving and tracking over a first frequencyband and for transmitting over a second frequency band as defined inclaim 8 additionally including means coupled to said separate waveguidescorresponding to said TE (45) and TM modes in said common circularWaveguide for generating an elevation error signal for a verticallypolarized signal in said common circular waveguide.

12. The feed system for receiving and tracking over a first frequencyband and for transmitting over a second frequency band as defined inclaim '8 additionally including means coupled to said separatewaveguides corresponding to said TE (45) and TM modes in said commoncircular Waveguide for generating an azimuth error signal for ahorizontally polarized signal in said common circular waveguide.

References Cited UNITED STATES PATENTS 12/1960 Lewis 333-11X 9/1966Lewis 33311 US. Cl. X.R. 333-21; 343786

